NASA's Mission Control has revealed the problem that prompted the early end of the latest spacewalk: Water in one of the astronaut's 35-year-old spacesuits.

Expedition 38 Flight Engineer Rick Mastracchio and fellow astronaut Michael Hopkins ran into trouble while they were conducting an urgent repair outside the International Space Station during a spacewalk that lasted five hours and 28 minutes.

The spacewalk ended short of its anticipated six-and-a-half-hour time frame when Mastracchio, the lead spacewalker, began complaining about chilly temperatures in his space suit.

The seven-time spacewalker said his feet were cold during at least part of the nearly five-and-a-half-hour walk and at times had to re-adjust temperature controls in his suit.

Even before the emergency repair mission began, NASA acknowledged it was working with aging spacesuits, which were designed in the same era of the space shuttle.

"Because the suits are 35 years old we review the hazards every so often as a matter of course," NASA's ISS Program Manager, Mike Suffredini told ABC News Radio.

The astronauts began working at 7:01 a.m. ET on Saturday to replace a degraded ammonia pump module associated with one of the station's two cooling loops that keep internal and external equipment cool, NASA said.

The engineers raced to successfully remove, ahead of schedule, the cooling pump that has jeopardized operations aboard the ISS since it broke on Dec. 11.

The 780-pound pump is about the size of a double-door refrigerator and difficult to handle, with plumbing full of toxic ammonia, The Associated Press reported. Flight controllers tried but failed to fix the bad valve through remote commanding.

During the repairs, the astronauts communicated with Mission Control Houston about the procedure. NASA's website offered the public a live video feed showing the astronauts and Mission Control.

At one point, when Mission Control asked Mastracchio to extend the spacewalk, he balked.

"My vote would be to call it for today, but it's up to you guys if you really want to go out there," Mastracchio said.

The work of the two astronauts Saturday was part of a planned series of spacewalks to replace the ammonia pump module.

While the astronauts successfully removed the pump on Saturday, plans to replace it two days later have been delayed. The pair will now spend Christmas Eve trying to finish the work, after NASA cancelled Monday's spacewalk to investigate what caused the latest malfunction.

NASA says a third spacewalk would occur on Christmas Day if necessary to finalize the installation of the replacement pump module. It would be the first Christmas spacewalk for NASA.

Mastracchio has conducted six previous spacewalks, and holds the record for the 14th longest total number of spacewalking hours. He wore extra safety gear to prevent a recurrence of helmet flooding that nearly drowned an Italian astronaut last summer. This was Hopkins' first spacewalk.

The two astronauts received guidance on the spacewalk procedures from NASA astronauts Doug Wheelock and Tracy Caldwell-Dyson, who replaced the pump at the same location during three spacewalks in August 2010, NASA said.

ABC News' SUSANNA KIM, ABC News Radio and the AP Contributed to this report.